Astroparticle Physics 31 (2009) 24–36

Contents lists available at ScienceDirect

Astroparticle Physics
journal homepage: www.elsevier.com/locate/astropart

Measurement of some EAS properties using new scintillator detectors
developed for the GRAPES-3 experiment
The GRAPES-3 Collaboration: P.K. Mohanty a, S.R. Dugad a, U.D. Goswami a, S.K. Gupta a,*, Y. Hayashi b,
A. Iyer a, N. Ito b, P. Jagadeesan a, A. Jain a, S. Karthikeyan a, S. Kawakami b, M. Minamino b, S.D. Morris a,
P.K. Nayak a, T. Nonaka b, A. Oshima b, B.S. Rao a, K.C. Ravindran a, H. Tanaka a, S.C. Tonwar aab

Tata Institute of Fundamental Research, Mumbai 400005, India
Graduate School of Science, Osaka City University, Osaka 558-8585, Japana r t i c l e

i n f oArticle history:
Received 10 September 2008
Received in revised form 29 October 2008
Accepted 9 November 2008
Available online 27 November 2008
PACS:
96.50.S
96.60.sd
95.55.Vj
29.40.Gx

a b s t r a c t
The GRAPES-3 extensive air shower (EAS) array started operation with 256 scintillator detectors at Ootyin India. Each detector is viewed by a fast photomultiplier tube (PMT) mounted at a height of 60 cm above
the scintillator. However, for further expansion of the array, an alternative readout of the scintillator
using wave-length shifting (WLS) ﬁbers is employed. This resulted in improved performance with a larger
photon signal and a more uniform response. With the inclusion of a second PMT, the dynamic range forparticle detection has been increased to 5  103 particles m2. We now use plastic scintillators, developed in-house to cut costs. The measurement of the density spectrum, shows a power law dependencewith an index c = 1.57 ± 0.04. Using the zenith angle dependence of the density spectrum, an attenuation
length Ka = 98–106 g cm2 for the EAS is obtained. These measurements are found to be consistent withthe results reported earlier by other groups.
Ó 2008 Elsevier B.V. All rights reserved.

Keywords:
Cosmic rays
Extensive air shower
Particle detector
Scintillation detector

1. Introduction
The GRAPES-3 (Gamma Ray Astronomy at PeV EnergieS – phase3) is a high density extensive air shower (EAS) array designed for
precision study of the cosmic ray energy spectrum and its nuclear
composition using the muon multiplicity distribution [1] in the energy range from 3  1013 eV to 3  1016 eV. The experiment wasstarted with 256 plastic scintillator detectors (each 1 m2 in area)
deployed on a hexagonal pattern, at Ooty (2200 m altitude,
11.4°N, 76.7°E) in southern India in 2001. The scintillator detector
is viewed by a fast photomultiplier tube (PMT) in a conventional
conﬁguration; wherein the PMT is mounted with its photo-cathodeface at a height of 60 cm, above the scintillator. The entire assembly of scintillator and PMT is housed inside a highly reﬂecting trapezoidal shaped light-tight aluminum container [2]. The GRAPES-3
array also contains a large area (560 m2) tracking muon telescope[3] to detect and measure the muon component in the EAS. The
muon telescope is an effective tool, in the study of the nuclear* Corresponding author. Tel.: +91 9869439435; fax: +91 2222804610.
E-mail address: gupta@grapes.tifr.res.in (S.K. Gupta).
0927-6505/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.astropartphys.2008.11.004

composition of primary cosmic rays. The tracking muon telescope
has also proved to be an invaluable tool in the studies of the solar
ﬂares, coronal mass ejections and the subsequent Forbush decrease
events observed at the Earth [4,5].
The occurrence of the ‘knee’ in the cosmic ray energy spectrum
around 3  1015 eV is believed to be intimately related to the issueof cosmic ray origin. But even after several decades of study, a clear
understanding of the origin of the ‘knee’ is yet to emerge. Clearly
the data obtained with higher sensitivity and lesser uncertainty
in the estimation of the primary energy and the composition are
expected to provide a better understanding of this important feature of the high energy astrophysics.
An efﬁcient detector system should provide a large separation
between the PMT noise and the signal due to the charged particles
in the EAS, and should also have a uniform spatial response overthe entire area of the scintillator. In the original GRAPES-3 trapezoidal shaped detectors, the scintillation photons collected at the
PMT are, primarily due to diffuse reﬂection from the inner wallsof the container [2]. This is not an efﬁcient process, and results ina sizable loss of the signal. Most of the scintillator detectors had
a photon output [10 photo-electrons, even for the 5 cm thick

25

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

scintillators used. The geometry of the detector also resulted in alarge spatial non-uniformity in the signal across the detector anda variation 30% is observed in photon output from the center tothe edge of the detector. In view of these limitations, we haveinvestigated the use of wave-length shifting (WLS) ﬁbers [6,7] foran efﬁcient collection of photons.
Observation of the photon pulses with a decay time of a few
nanoseconds in plastic ﬁbers were reported in 1981 [8] and subsequently, extensive development of the plastic ﬁbers occurred. Useof WLS ﬁbers, where wave-length shifting cores ranging from several hundred microns to a few mm diameter, surrounded by single
or multiple layers of cladding of progressively lower refractive
indices were developed. The double clad WLS ﬁbers provide readout of the blue photon emitting scintillators in a detector, with thesubsequent re-emission of the photons in the green wave-length
using a 3-hydroxyﬂavone (3HF) type wave-length shifter present
in the core material [9]. A fraction of the re-emitted photons aretransported to the end of the ﬁber through total internal reﬂections. It is known that the double clad ﬁbers provide an improved
photon yield of up to 70% higher than the single clad ones, byincreasing the fraction of light trapped through total internal
reﬂections [10].
We have developed a new scintillator detector with signiﬁcantly improved performance, by a suitable placement of the
WLS ﬁbers in machined grooves on the scintillator surface. Since,
the spatial uniformity and the response time of the scintillator critically depends on the layout of the ﬁbers, the performance of thedetector has also been simulated using a Monte Carlo simulation
code ‘G3ScSIM’ developed by us. A direct comparison of the resultsfrom the simulations and the actual measurements has allowed, abetter understanding of the performance of the detector to emerge.
Three different layouts of WLS ﬁbers were evaluated using Monte
Carlo code, G3ScSIM. However, the ﬁnal conﬁguration has been
adopted on the basis of cost effective fabrication with excellent
uniformity and high photon output. The saturation of the PMTs
encountered at very high particle densities in the earlier GRAPES3 detectors, limited their use over a wider range of primary cosmic
ray energies. To overcome this limitation, the WLS ﬁber detectors
have been designed with two PMTs. The results of the tests andthe performance of the new detectors is summarized here.
The density distribution of the particles observed in an EAS
detector station provides a useful and simple method of probing
the energy spectrum of the primary cosmic rays. The integral density spectrum is deﬁned as the frequency N(q) with which the particle density exceeds a value q at a given location. To a ﬁrst
approximation the density spectrum N(q) is given by,NðqÞ ¼ K  qcattenuation length of (124 ± 11) g cm2 in Tibet, at an atmosphericdepth of 606 g cm2 [15]. The KASCADE collaboration has studied
the attenuation of the EAS in great detail at the sea level, and haveobtained a value of (100–120) g cm2 for this parameter [16].
Although, a direct comparison of the above two experimental results is rather difﬁcult, owing to the different altitudes of observation and radically different methods of data analysis employed.
However, the fact that the magnitude of the attenuation length
are same within stated errors, indicates that it is very usefulparameter to describe the attenuation of EAS in the atmosphere.
2. The experimental setup
A special experimental setup has been erected, for studying theperformance of the scintillator detectors being developed. A sketchof the arrangement used in this setup, consisting of three sets oftrigger scintillators is shown in Fig. 1. The two scintillator hodoscopes are placed, directly above and below the scintillator detector under development. Each hodoscope consists of a total of 10
strips of plastic scintillators, each strip is 50 cm long with across-sectional area of 10 cm  2 cm. The 10 strips are divided into
two groups of 5 strips each. The ﬁrst group of 5 strips labeled U1
through U5, are placed adjacent to each other to form a layer, covering an area of 50 cm  50 cm, seen as the top layer in Fig. 1. The
second group of 5 strips labeled L1 through L5, are also similarly
deployed, but along a direction orthogonal to the ﬁrst group. Each
strip is viewed by an individual 5 cm diameter, fast PMT (model
ETL-9807B) coupled directly to the strip. To avoid the cross-talk,each strip is wrapped in aluminum foil and then inside a layer ofblack plastic sheet. The high voltage applied to each PMT is adjusted, to make it fully sensitive to single muons. Thus, each hodoscope records the passage of a through going muon by the twogroups of scintillator strips along two orthogonal, namely X- andY-directions to a precision of 10 cm  10 cm. A total of 25 localized
cells, of constant cross sectional area (10 cm  10 cm) are obtained
from each hodoscope as is shown in Fig. 1. A vertical separation of
50 cm between the two hodoscopes allows the direction and thelocation of each muon to be determined.
In addition, another plastic scintillator (50 cm  50 cm  5 cm
in size) is placed below the lower hodoscope, inside an inverted,
trapezoidal shaped cone, with inner surfaces painted black. The
PMT viewing this scintillator is placed at the bottom of the cone,
which detects only direct photons, thus providing signiﬁcantly better timing.

ð1Þ

However, the actual measurements have shown that c is only
approximately constant and it shows a very weak dependence on
the detected particle density. Nevertheless, for most purposes a
constant value of c provides an adequate description of the data.
If all EAS have an identical lateral distribution, the density spectrum
represented by Eq. (1) implies a similar size spectrum of EAS [11].
The measurements of the density spectrum have shown a clear
power law dependence as outlined above. The density spectrum obtained from a plastic scintillator detector at an altitude of 2770 m at
Mt. Norikura showed a dependence of N(q) = K  qc; wherec = 1.55 ± 0.05 was obtained in the density range 50–104 particles
m2, which progressively increased to 2.0 at the densities beyond
2  104 particles m2 [12].
The zenith angle distribution of the EAS has been studied for along time to measure the attenuation of the EAS in the atmosphere[13,14]. The zenith angle distribution of the EAS for different particle densities has been used to estimate the attenuation lengthof the EAS by the ARGO–YBJ collaboration. They have reported an

Upper Strips
U1 − U5

Hodoscope−1

Lower Strips
L1 − L5
Test Scintillator

Hodoscope−2

PMT

Cone Detector

PMT
Fig. 1. A sketch of the experimental setup with two identical hodoscopes placed,
above and below the test scintillator along with the trapezoidal shaped detector.

26

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

The muon trigger is generated as described below. As shown in
Fig. 2, the output pulses from each of the PMTs coupled to the 10
strips from the 2 layers of a hodoscope, are discriminated to produce logical output pulses. Next, a logical OR of the discriminatoroutputs, from the 5 strips in a layer called OR1 is generated whichindicates the passage of a muon through layer 1 in hodoscope 1.
Thus the 4 layers of the scintillator strips from the two hodoscopes
provide 4 independent signals OR1, OR2, OR3, OR4 during the passage of a muon. Finally, a 4-fold coincidence of OR1, OR2, OR3, OR4
is taken to generate the initial muon trigger l0. The output of the
PMT from the trapezoidal shaped detector is also discriminatedand its coincidence is taken with l0 after a small delay, to generate
trigger l1. Signal l1 provides faster timing, because the trapezoidal
shaped detector triggers only on direct photons which have significantly smaller time spread. Signal l1 is subsequently also used as
the GATE for the charge integrating analog to digital converters(ADCs) and as the START for the time to digital converters (TDCs).
Since l1 is generated by a 5-fold coincidence, it provides a relatively clean muon trigger, free from the noise in the PMTs.
The passage of the muons through the speciﬁc scintillator strips
in the two hodoscopes is obtained, by latching the discriminatoroutput pulses from all 20 strips as a ﬂag by the use of a 20-bit ﬂag
latch module, where the corresponding bit for the strip is set to thelogical state ‘1’ if that strip is hit, otherwise it is set to the state ‘0’.
Normally a muon would results in at least 4 strips being hit fromthe 4 layers of scintillators in the two hodoscopes. A small fractionof the total events record hits in more than one strip in a single
layer, which might be due to the passage of an inclined muon,
through two adjacent strips or due to the production of a knockon electron by the parent muon. Events with such multiple hits
are rejected during the subsequent data analysis.

400 ns
ADC
GATE

Test
Detector

The information on the scintillator strip hits are used to localize
the muons to within cells of 10 cm  10 cm in size, as shown in
Fig. 1. However, only near vertical muons are selected for further
analysis. To obtain a reasonably good statistics, the data are accumulated over a period of 7 days. The ADC and the TDC distributions
are obtained for each of the 25 position cells (10 cm  10 cm insize). These ADC and TDC distributions are then compared withthe results of the Monte Carlo simulations. The PMT viewing thetest detector has been calibrated by measuring its response to single photons, which is shown in Fig. 3. The observed ﬁrst photoelectron peak has been ﬁtted to a Gaussian function. The widthof this ﬁt is used in the subsequent Monte Carlo simulations togenerate the PMT response.
In the present study, Kuraray double clad WLS ﬁbers of type Y11(200) [17] have been used. These 1 mm diameter ﬁbers consist of acentral polystyrene core (refractive index g = 1.59) with two outercladdings of progressively lower refractive indices. The inner cladding is polymethyl-methacrylate (PMMA) with g = 1.49 and theouter cladding is ﬂuorinated polymer (FP) having g = 1.42. All ﬁbers
are cut to a length of 1.3 m. To place the WLS ﬁbers inside the scintillator, 1.5 mm wide and 2 mm deep grooves have been machinedon the surface of the scintillator (50 cm  50 cm  2 cm). Three different groove designs named, ‘parallel’, ‘matrix’ and ‘r’ have been
investigated. The layout of these 3 designs are schematically shownin Fig. 4. Since the parallel and matrix grooves are straight, they aremachined using an ordinary milling machine. However, the rgrooves require the use of a CNC milling machine.
For the parallel and the matrix conﬁgurations, a total of 12
grooves with equal inter-groove separation are machined. An inter-groove separation of 4.0 cm is used for the parallel grooves(50 cm long) as shown in Fig. 4a. For the matrix grooves the separation is 9.6 cm, for the two mutually orthogonal sets of 6 grooves(50 cm long), as shown in Fig. 4b. A total of 16 r grooves are machined, each 41 cm long as seen in Fig. 4c. The total length of the
WLS ﬁbers embedded in the scintillator is 900 cm in the cases ofthe parallel and the matrix groove conﬁgurations. The WLS ﬁber

TDC

Disc.

START
GATE
GEN.

OR
OR1
OR2
OR3
OR4

OR
Flag
Latch
Module

Disc.

AND

AND

50000

Pedestal

μ0

μ1

30000

1 pe
20000

0

Disc.

Fig. 2. A schematic diagram of the trigger and signal processing setup used in themeasurements.

a

40000

10000

Trigger
50 ns

b

10

c

50cm

50

cm
50

2cm

2cm

20

30
40
ADC Counts

50

60

Fig. 3. Single photo-electron response of the PMT used in the test scintillator. A
Gaussian function is ﬁtted to the ﬁrst photo-electron peak.

cm

50cm

0

50cm

cm

Cone
Detector

Fit Statistics
Constant
1.749e+04
Mean
23.74
Sigma
4.509

50

L1
L2
L3
L4
L5

Disc.

Number of Events

U1
U2
U3
U4
U5

2cm

CookieFig. 4. The three groove layouts on the scintillator surface with embedded WLS ﬁbers (a) parallel, (b) matrix, and (c) r.

27

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

3. Monte Carlo simulations (G3ScSIM Code)Prior to the actual experimental measurements, a Monte Carlosimulation code ‘G3ScSIM’ [19] has been developed to get anunderstanding of the response of the scintillator for different ﬁber
conﬁgurations. The G3ScSIM is basically a photon tracking code, fora plastic scintillator of rectangular geometry, with embedded WLS
ﬁbers of cylindrical geometry. In this code, the trigger conditions
are kept identical to those in the experiment with cosmic raymuons, as shown in the Fig. 1. The important input parameters
used in G3ScSIM are listed in Table 1. Some of the simpliﬁedassumptions made in G3ScSIM are: (i) the scintillator response ishomogeneous, (ii) the scintillator photons are monochromatic,and (iii) the WLS ﬁbers are in optical contact with the scintillator
surface, over an area projected by it. A brief description of someof the relevant features of G3ScSIM code are summarized below
and described in greater detail elsewhere [19].
3.1. Basic G3ScSIM code
The detector setup modeled in G3ScSIM code, is identical to thatin Fig. 1. The origin of the coordinate system is selected at the corner of the test detector and the geometry is deﬁned through its extreme coordinates (Xmin,Ymin) to (Xmax,Ymax). The cosmic ray muons
are generated over the surface of the top hodoscope (Fig. 1) withrandom coordinates x, y and arrival direction; zenith angle h(0 6 h 6 60 °), azimuth angle / (0 6 / 6 360 °). By projecting the
trajectory of the muon, its location on the test detector, and on
the other triggering detectors is calculated. A trigger is generated,
only if a muon passes through all the ﬁve triggering detectors.
The energy loss dE/dx of the muon, in the test scintillator is
calculated using the Landau distribution [20,21] based on an algo-

Table 1Monte Carlo simulation parameters.
Scint. Refractive index (gsc)
WLS (gwls) Fx core 1.59
In. cladOut. clad
Scint. decay time
WLS ﬁber decay time
Scint. attenuation length
WLS attenuation length
Tyvek reﬂectivity
ETIR
PMT time jitter (FWHM)
Mean energy loss/photon

1.59
1.49
1.42
1.7 ns
6.1 ns100 cm350 cm90%
93%
2.2 ns100 eVrithm from the CERN library [22]. The computed energy loss distribution of vertical muons in a 1 cm thick scintillator is shown in
Fig. 5, assuming a mean energy loss of 1.89 MeV/g cm2, for relativistic muons [23]. A production rate of one short wave-length(blue) photon per 100 eV energy loss is assumed. The blue photons
are generated along the track of the muon, every 0.1 mm, with theemission angles h and /, selected randomly from an isotropic angular distribution. The emission time of each photon is calculated
from an exponential distribution with a decay time of 1.7 ns forthe scintillator, as measured by us earlier.
The propagation of the blue photons in the scintillator istracked, by a ray tracing method, governed by the laws of geometrical optics [24]. The incidence of a photon on the scintillator surface is determined, from among the six surfaces of the scintillator.
At every reﬂection from the scintillator surface, three possibilities
exist, namely; (i) the absorption of the photon in the scintillator,(ii) the interception by a WLS ﬁber, and (iii) the reﬂection fromthe scintillator surface. To determine absorption of a photon in
the scintillator, the total path-length traversed is compared withthe survival path-length for that photon, using the Monte Carlotechnique.
Normally a photon incident on the scintillator surface at an angle greater than the critical angle, would undergo total internal
reﬂection from a ﬂat surface. However, the scintillator surface is
not perfectly ﬂat and any departure from the ﬂatness would resultin the escape of the photon. This effect is taken into account byassuming an effective total internal reﬂectivity (ETIR) <100%. The
exact value of ETIR is determined empirically as described below.
The Tyvek wrapping of the scintillator provides an efﬁcient and diffuse reﬂecting surface of known reﬂectivity, for the photons escaping from the scintillator back into the scintillator. The photon
reﬂection probability from the diffuse reﬂector is calculated using
the Lambert’s cosine law [25], where the angle of reﬂection is independent of the incident angle. However, when the scintillator iswrapped inside a black plastic sheet, the reﬂectivity from its surface of escaped photons is assumed to be zero.
To determine the incidence of a photon on the WLS ﬁber, we assume the area projected by the ﬁber on the scintillator surface as
the overlap area. If a photon is incident on the overlap area, it isdeemed to be absorbed by the ﬁber. Subsequently, a new longer
wave-length (green) photon is generated with a wave-length distribution derived from the emission spectrum of WLS ﬁber, andtracked in case of trapping in the ﬁber, using critical angle condition. At this point, a new start time is generated from the measured
decay time (6.1 ns) of the WLS ﬁber. This in turn is added to the

20000

Peak = 1.59 MeV

Number of Events

are restrained from any movement by applying tiny strips of adhesive aluminum tape just above the groove. A total of 18 ﬁbers havebeen used and placed in alternative grooves as single and two ﬁbers,
respectively. The total length of WLS ﬁbers is 656 cm for the r conﬁguration with only one ﬁber in each groove as shown in Fig. 4.
The entire scintillator and WLS ﬁber assembly is wrapped inside
two layers of Tyvek paper [18], which acts as an efﬁcient but diffuse reﬂector, resulting in a substantial enhancement in the number of the photons collected. Tyvek has been chosen for its highreﬂectivity, chemically inert nature and excellent mechanicalstrength. Since only a small fraction of the total length (41 or
50 cm) of WLS ﬁber is embedded inside the groove, the remainder
is used for its smooth and safe bending, en-route to the PMT. The
free ends of the WLS ﬁbers are bunched together and glued into
an acrylic disk called a ‘cookie’ which is ﬁnally coupled to the
PMT using an optical glue. The entire assembly is placed inside alight-tight aluminum tank. The experimental conditions are kept
unchanged during the tests with different groove designs.

15000

10000

5000

0

1

2
3
4
Energy Loss (MeV)

5

6

Fig. 5. Energy loss distribution of vertical muons in a 1 cm thick scintillator. Mean
energy loss = 1.89 MeV. A Landau distribution ﬁt gives a peak at 1.59 MeV.

28

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

earlier transit time of the blue photon, from the time it was generated in the scintillator to the time of its absorption in the ﬁber. A
trapped green photon is propagated, until it is absorbed in the ﬁber
due to attenuation or if it reaches the collection end of the ﬁber.
The green photons propagating in the direction opposite to thatof the PMT are assumed to be lost. The green photons reaching
the PMT are converted into photo-electrons at the photo-cathodeusing the wave-length dependent quantum efﬁciency of the PMT.
The amplitude and the response time of the PMT are convoluted
to generate the PMT pulse. For each muon, the recorded information includes the X, Y coordinates on the triggering and test scintillators, also the photon statistics such as the number of photons (i)produced in the scintillator, (ii) escaped from the scintillator, (iii)lost due to the attenuation, (iv) absorbed in the WLS ﬁber, (v) escaped from the ﬁber, and ﬁnally the number of the photo-electrons
and their arrival times at the PMT.

only a small fraction is eventually received by the PMT. The simulations show that 86% of the produced (blue) photons are either
absorbed or escape from the scintillator. The mean path-length traversed by the blue photons is 51 cm and the mean number ofreﬂections is 14 inside the scintillator. Therefore, 14% of blue
photons manage to enter the WLS ﬁber, which after initial absorption are re-emitted as green photons. Furthermore, because of thelarge value of the critical angle, most of the green photons escapefrom the WLS ﬁbers, and only 14% of the photons are trapped.
These photons travel along both directions in the ﬁber. The mean
path-length traversed by green photons is 112 cm and the mean
number of reﬂections is 430 prior to reaching the PMT. Finally,
only 0.45% of the total photons produced in the scintillator,
namely 208 photons arrive at the PMT.

3.2. Effective total internal reﬂectivity (ETIR)The main design goal for the new detector is to efﬁciently record shower particles over a large dynamic range (1–10,000 particles), to cover a wide range in the energy spectrum of the primarycosmic rays from 1013, where the particle density at the core israther small; to 1017 eV where densities may reach several thousand particles m2. Efﬁcient detection of a single or a few particles
is ensured by designing a detector with high photon yield and good
spatial uniformity. The high photon yield also implies that the PMT
may be operated at a relatively lower voltage, thereby reducing thenoise and increasing its life. The PMTs are typically operated at again of (3–5)  106, which is within the recommended region speciﬁed by the manufacturer. However, at high particle densities, theanode current become very large and the PMT response become
non-linear. For the ETL-9807B PMT used here, the response becomes non-linear at a peak anode current of 50–150 mA. Thistranslates into the onset of the non-linearity at J 50 particles
and of saturation at 100–200 particles. This introduces error in
the estimation of the various shower parameters. Although, one
can use the signal from an earlier stage of the dynode chain to
avoid saturation, we prefer to employ a second PMT to enhancethe dynamic range of our detectors.
As described below, through a study of the; (i) photon output,(ii) uniformity and (iii) the time response for different groove designs, we ﬁnd that the simple and inexpensive parallel groove conﬁguration meets our design goals. Using 4 scintillator tiles, a 1 m2
detector has been fabricated. The side-view of a complete dualPMT WLS ﬁber detector is shown in Fig. 6. The single-PMT ﬁber
detector is identical to a dual-PMT detector, except that it is operated with only one high-gain PMT. The photons from each
50 cm  50 cm scintillator tile are collected by 18 ﬁbers for thehigh-gain PMT, whereas only 6 ﬁbers are used for the low-gain
PMT. The groups of 18 and 6 ﬁbers for the high- and low-gain PMTs
are placed inside 12 grooves, as has been described in the Section
2. Two groups of ﬁbers from the 4 scintillators are coupled to two

A Monte Carlo simulation can reproduce the experimental data
only if the detector parameters describing its performance are sufﬁciently close to their true values. These parameters include thescintillation efﬁciency, the surface reﬂectivity, the attenuation
length, and the decay times etc. Many of these parameters havebeen directly measured by us or are available from the literature.
The remaining parameters have to be obtained by assuming some
initial values and then ﬁne tuning those values by matching thesimulation results with the experimental data for a particular conﬁguration of the test detector. The results can then be crosschecked by carrying out another set of measurements with a different conﬁguration of the test detector.
To ﬁx the free parameters in the simulations, the parallel groove
design, with and without the Tyvek wrapping was modeled. The
only free parameter used in these simulations is the ETIR. The initial value of the ETIR is taken to be 100%, implying that the scintillator surfaces to be perfectly ﬂat. The resultant photon distribution
from the simulations disagreed with the experimental data. It isobserved that the simulated ratio of the mean number of photons
with and without the Tyvek wrapping is 2.2, while the experimental value is 3.3. It is observed that this ratio increases with decreasing value of ETIR. A value of ETIR = 93%, gives a nearly perfectagreement with the experimental data. This value of the ETIR has
been used for all subsequent simulations.
3.3. Photon statistics
The muons suffer a mean energy loss of 1.89 MeV/cm in a scintillator. However, when due account is taken of the muons arrivingfrom different directions that trigger the setup, the mean energy
loss becomes 4.6 MeV in the 2 cm thick scintillator used. This results in production of a large number of photons (46,000), but

4. The detector design

Pulse
High Voltage
High Gain PMT
Aluminum cylinder
to house PMT and Base
Aluminum box

Scintillator tile

Low Gain PMT
Cookie holder

WLS fibers

Fig. 6. Side-view of dual-PMT ﬁber detector. Total number of ﬁbers from the scintillator to high-gain PMT is 72 and to low-gain PMT is 24.

29

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

PMTs, through two separate cookies; the ﬁrst containing 18  4 =72 ﬁbers to the high-gain PMT, and the second 6  4 = 24 ﬁbers
to the low-gain PMT.
The PMT is placed inside an aluminum cylinder to shield it fromexternal electrical noise. The two PMTs are vertically mounted at
the center of an aluminum tank of dimension 112 cm  103 cm 
5 cm as shown in Fig. 6. This detector is more compact, lighter,and easier to maintain, as compared to the old trapezoidal design.
The fabrication of the dual-PMT ﬁber detectors started in 2004,
and about 100 such detectors have been fabricated and successfully deployed in the array. These detectors are placed every
16 m, starting from the center of the array, by replacing the corresponding old trapezoidal shaped detectors.

a

6000

4000

2000

b

Number of Events

6000

5. Discussion
5.1. Photon yield
The reﬂecting surfaces play a key role in obtaining a higher photon yield and a better uniformity. Thus a quantitative comparisonhas been done both, without and with a reﬂecting Tyvek wrapping,using a scintillator with parallel grooves. The measured photo-electron distributions are shown in Fig. 7: (a) with a black plastic wrapping to prevent any reﬂection of the photons escaping from thescintillator back into it and, (b) with two layers of Tyvek wrapping
to ensure efﬁcient reﬂection of the escaped photons back into thescintillator.
Both distributions are asymmetric and display the characteristic
Landau tail. Therefore a convoluted Landau with a Gaussian ﬁt(http://root.cern.ch/root/html/examples/langaus.C.html) has been
used, to estimate the peak values. The peak values provide a measure of the scintillator photon output. The peak is at 6.4 photoelectrons with the black plastic sheet wrapping and 20.5 photoelectrons with Tyvek wrapping. Thus the ratio of the photon output
with the Tyvek and the black plastic wrapping is 3.2 for the scintillator with parallel grooves.
A comparison of the photo-electron distributions for the parallel, the matrix, and the r groove designs for the experimental data
and the Monte Carlo simulations are presented in Fig. 8, and the results are also summarized in Table 2. Here it needs to be empha-

4000

2000

c

6000

4000

2000

0

20

40
60
Photo-electrons80

Fig. 8. Photo-electron distributions: (a) parallel groove, (b) matrix groove, and (c) rgroove. Experimental data shown by dashed line and Monte Carlo results bycontinuous line.

Table 2
Photo-electron yield of scintillator for different WLS ﬁber conﬁgurations.

10000

a

8000

Npe = 6.4
6000

GrooveFiber-length (cm)Photo-electrons

Parallel
Matrix

900
900
656

20.5
21.7
17.9

r

Number of Events

4000
2000

b

5000
4000

Npe = 20.5

3000
2000
1000
0
0

10

20

30
40
50
Photo-electrons

60

70

80

Fig. 7. Photo-electron distribution from muons in parallel grooved detector
wrapped in (a) black plastic, (b) two layers of Tyvek. Fits to a convoluted Landauwith Gaussian yields, peak at (a) 6.4 and (b) 20.5 photo-electrons.

sized that the simulation parameter ‘ETIR’ has been extracted,using only the parallel groove data and then subsequently used
for the matrix and the r grooves. An excellent agreement between
the photo-electron distributions from the data with the Monte Carlo simulations for the remaining two designs namely the matrix
and the r is seen in Fig. 8. Further, it can be seen from the Table
2 that the photo-electron yield in the case of the parallel grooveis comparable to that of the matrix groove, whereas it is signiﬁcantly lower in the case of the r groove. The embedded length of
the ﬁbers in the r groove is 27% less, as compared to the othertwo groove designs, however, the photo-electron yield in the rgroove is 13% less as compared to the parallel groove and 18% lesscompared to the matrix groove. Thus the photo-electron yields perunit length of the embedded ﬁber, for the three groove designs arecomparable to within 10%, irrespective of the shape and the layout of the grooves. Based on these results, it has been decided toP.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

employ the parallel groove conﬁguration, since it is economical to
implement and the assembly of the detector is also relatively
simpler.
To measure the photo-electron yield as a function of the number of the WLS ﬁbers, we used a scintillator with parallel grooves
with 3, 6, 9, 12 and 18 ﬁbers. Different number of ﬁbers wereaccommodated in the 12 grooves on the scintillator surface, by distributing them in a uniform manner. The photo-electron yield foreach of these ﬁve conﬁgurations is also simulated. The experimental data and the simulation results are shown in Fig. 9. It is seenthat the photo-electron yield increases linearly with the numberof ﬁbers and there is good agreement between the data and thesimulations. For the ease of estimating the number of detected
photo-electrons, a linear relation is ﬁtted to the experimental databy

Npe ¼ a þ bN F ;ð2Þwhere Npe is detected number of photo-electrons, NF the number of
WLS ﬁbers. The linear ﬁt to the experimental data yields the valuesof the; intercept a = (0.1 ± 1.0) and the slope b = (1.12 ± 0.09). As
seen from Fig. 9 this ﬁt also provides an adequate description of
the simulation results.

T

s to

er
Fib

80
70
60
50

ers

Fib

Number of Photo-electrons

0

5

10

15

Number of WLS Fibers
Fig. 9. Variation in detected photo-electrons as a function of number of WLS ﬁbers:(a) N experimental data, (b)  Monte Carlo results. Dashed line is linear ﬁt toexperimental data.

0

10

20

50

30
m)
X (c

M
to P

b

90

70

5

30
Y (c 20
m)
10

40

100

The photon output across the scintillator surface for the parallel
grooves is shown in Fig. 10 as Lego plots, for the two cases of scintillator wrapped inside; (a) black plastic sheet, and (b) Tyvek paper
respectively. In these plots, the vertical axis represents the relative
photon yield in one of the 25 positions, and the highest value isnormalized to 100. The Y-axis represents the longitudinal directionof the ﬁbers in the scintillator, whereas the X-axis represents thetransverse direction. In the case of the black plastic wrapping(Fig. 10a) two clear trends are observed namely a decrease in thephoton output; (i) along the ﬁber, away from the PMT and (ii) fromthe central area towards the edge, along the transverse direction isseen. The ﬁrst trend is due to the attenuation of the photons in the
WLS ﬁbers. This is because farther is the location of a muon fromthe PMT, longer is the length of the ﬁber that a photon has to travel
prior to its detection, resulting in enhanced attenuation. The second effect may be due to the fact that, as the muon passes closer
to the edge, the escape probability of the produced photons increases. With the Tyvek wrapping, the response is far more uniform as seen in Fig. 10b. Only a very small decrease is seen along

10

40

T

80

15

a

90

5.2. Detector uniformity

20

PM

100

Normalized Photon Output

30

60
50

40

30
Y (c 20
m)
10

0

30
20 m)
10
X (c

40

50

Fig. 10. Normalized photon output for parallel grooves: (a) with black plastic
wrapping, (b) with two layers of Tyvek wrapping.

the longitudinal direction of the ﬁbers. The response near the edge
is also higher than in the central area. This behavior is opposite of
the case with the black plastic wrapping. This may be due to the reentry of the escaped photons at the edges, due to the reﬂections
from the Tyvek surface, back into the scintillator. The Tyvek wrapping also increases the photon yield by a sizable factor (3.2) besides improving the uniformity of the detector response.
The variation in the response, across the scintillator surface forall the three groove designs with Tyvek wrapping, are shown in
Fig. 11 as Lego plots. In the case of the parallel groove, a small decrease in the photon output is observed, from near the PMT end to
the far end of the ﬁber. In the matrix groove, a similar trend with
reduced magnitude is seen. However, in the case of the r groove,no such variation is observed. Also no transverse variation in thephoton output is observed for the matrix and r grooves, whereas
for the parallel groove the edge response is slightly higher thanfrom the central area as stated above.
The photon output variation in the longitudinal direction of the
ﬁber is calculated by averaging the data in the transverse direction
and the results are presented in Fig. 12 for a comparison with thesimulations. In Fig. 12a for the parallel grooves both simulation anddata show a decrease 5% in the photon output, over a distance of
50 cm, on the other hand, in the case of matrix grooves (Fig. 12b)
there is a smaller drop (1–3)% in the central region as comparedto the edge positions for both data and simulations. However,the situation is exactly reversed in the case of the r grooves(Fig. 12c), where a small increase (1–3)% is seen.
The root mean square (rms) non-uniformity for the threegroove designs is calculated separately, for both the data and thesimulations and these are summarized in Table 3. The simulated

31

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

a

T

ers

Normalized Photon Output

Fib

MT

M
to P

oP
rs t

b

Fib

ers

e

Fib

T

to P

ers

MT

Fib

100

100

100

90

90

90

80

80

80

70

70

70

60
50

40

30
Y (c 20
m)
10

0

10

30
20 cm)
X(

50

40

60
50

40

30
Y (c 20
m)
10

0

10

30
20 cm)
X(

40

50

60
50

40

c

M
to P

30
Y (c 20
m)
10

0

10

30
20 cm)
X(

40

50

Fig. 11. Normalized photon output for (a) parallel, (b) matrix, and (c) r grooves.

(a)

100

Table 3
Uniformity in different groove patterns.
Groove type

RMS non-uniformity (%)
ExperimentMonte Carlo

Parallel
Matrix

2.7
2.1
3.5

2.0
1.6
3.3

95
90

r
85

the rms non-uniformity is less than 4% for all three groove designs,
well below our acceptable level of 5%.

Relative Photon Output

80
(b)

100

5.3. Time response

95
90
85
80
(c)
100
95
90
85
80
10

20
30
Distance (cm)40

Fig. 12. Normalized photon output variation from near PMT to far end of the ﬁberfor (a) parallel, (b) matrix, and (c) r grooves. h experimental data and 4 Monte
Carlo simulation.

values of rms non-uniformity is always slightly smaller (0.2–0.7)%,
than the corresponding experimentally measured value for all thethree designs. This is not surprising, in view of the fact that manyof the assumptions in the simulations are only approximations to
the real world. For example, any imperfections on the surface ofthe scintillator or any variations in its density, would result in enhanced rms non-uniformity in the experimental data. However,The distribution of the time difference between the test scintillator and the trapezoidal shaped trigger detector is obtained frommuons. It is measured using the setup shown in Fig. 1 and the trigger scheme shown in Fig. 2. The trapezoidal shaped detector at thebottom which has the fastest time response, provides the STARTand the test scintillator provides the STOP to the TDC. In the subsequent analysis the entire area (50 cm  50 cm) of the detector
is used, thereby incorporating the time differences, due to the
propagation of the photons inside the scintillator as has been explained earlier in Section 2.
The measured time distributions for the three groove designs
are shown in Fig. 13 by a dashed line and compared with the simulated distributions on the same plot which is shown by a continuous line. For this comparison the peak positions of the data andthe simulated distribution were artiﬁcially aligned. This is acceptable because a ﬁxed time offset has no physical signiﬁcance. A reasonable agreement is observed between the widths of the time
distributions for the data and the simulations. For the parallel
grooves the full width at half maximum (FWHM) is 6.5 and
6.1 ns (Fig. 13a), for the matrix grooves the FWHM is 6.4 and
6.0 ns (Fig. 13b), and for the r grooves the FWHM is 6.3 and6.3 ns (Fig. 13c), for the experimental data and simulations, respectively. The values of the FWHM for the three groove designs agree
to within ±0.2 ns. This shows that the time resolution is largely
independent of the groove design.
The position dependence of the arrival time from the simulations indicates that the time difference between the far- and thenear-end positions along the ﬁber is 1.7 ns in the case of parallel
grooves and 1.0 ns for the matrix grooves. This may be understoodas the transit time difference along the length of the ﬁber. The photons generated near the PMT will arrive earlier than those from theP.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

far end, resulting in a ﬁnite transit time difference. However, forthe case of the r grooves, no variation in transit time is expected
and none is observed.
A low photon detection threshold and a high photon yield areconsidered necessary for getting a good time resolution in a scintillator detector [26]. The effect of these two factors, on the time resolution has been quantitatively studied, through simulations. For
this purpose the detector response, for the number of embedded ﬁbers NF = 6, 9, 12, 18 has been simulated. From these simulations
we estimate the mean number of photo-electrons (Npe) to be 6.4,
10.0, 13.4, and 20.6 for 6, 9, 12, and 18 embedded ﬁbers respectively. The results of this study are shown in Fig. 14. The 4 plots
in Fig. 14, clearly show that the time resolution improves withdecreasing threshold. It is also clear that for a given threshold,increasing the number of ﬁbers (NF), results in a better time resolution. For smaller values of NF, the time resolution changes linearly with the threshold, whereas the change is somewhat slower
at higher values of NF. For NF = 6, the increase in the time resolution
is 1.45 ns/Npe, whereas it is only 0.13 ns/Npe for NF = 18. Thus thetime resolution becomes less sensitive to the threshold for higher
values of NF. The time resolution can be improved either byincreasing the photon output or reducing the threshold. However,
the threshold level of the discriminator can not be arbitrarily reduced, due to the increased contribution to the count rate, fromthe PMT and other sources of noise.

9

NF=6
8
7
Time Resolution σ (ns)32

4

N F=18

3

1
0

0

2

4
6
8
Threshold (Npe)1012Fig. 14. Dependence of time resolution on photo-electron threshold as derived
from simulations. Each plot is for a different number of embedded WLS ﬁbers.

Encouraged by the good agreement in the time response observed between the data and the simulations, an effort has been
made to understand the effects of various experimental factors
inﬂuencing the time resolution, based on the simulations. The time
resolution of the detector may be parametrized as follows:

rtotal ¼

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
r2ssc þ r2stat þ r2sWLS þ r2PMT

ð3Þ

where rssc is the contribution to time resolution due to the decay
time of the scintillator, rstat due to the statistical ﬂuctuation in
the number of detected photons, rsWLS is due to the decay time of
WLS ﬁber, and rPMT is the contribution due to the transit time
spread in the PMT. The Monte Carlo simulations have been performed for the parallel groove conﬁguration with Tyvek wrapping.
First, rstat is computed by setting all other decay times namely,
ssc, sWLS and rPMT to zero in the simulations. The value of rstat isfound to be 1.4 ns. Similarly rssc is determined to be 1.0 ns and ﬁnally rsWLS is calculated to be 2.2 ns. These results imply that thelargest contribution to the time resolution is due to the decay timeof WLS ﬁber. This is not very surprising because the measured decay
time of WLS ﬁber at 6.1 ns, as mentioned earlier, is the biggest contributor to the time resolution.

3000

2000

1000

0

b
Number of Events

N F =12

5

2

4000

a

NF=9

6

3000

2000

5.4. The detector performance

1000

0

c
3000

2000

1000

0

5

10

15
20
Time (ns)25

30

Fig. 13. Time response of the scintillator for (a) parallel, (b) matrix, and (c) r groove.
Experimental data is shown as dashed line and simulations by continuous line.

To compare the performance of the newly developed ﬁber
detector with the old trapezoidal detector, about three months of
the EAS data (1 March–29 May 2005) have been analyzed for bothtypes of detectors. The PMTs used in the trapezoidal detectors areoperated at a gain that is comparable to that of the high-gain PMT
in the ﬁber detectors. During this period only 20, dual-PMT ﬁber
detectors were operating and these were distributed starting fromthe center of the array, to right up to the 6th ring in the array. The
GRAPES-3 air shower trigger system, the trigger criteria and therecording of the shower information have been described in detailelsewhere [2]. For the high-gain PMT in a dual-PMT ﬁber detector
and the trapezoidal detector PMT, both the ADC and the TDC information is recorded, while for the low-gain PMT (dual-PMT detector), only the ADC data are recorded. The ADC data are converted
into an equivalent number of minimum ionizing particles for eachshower. Since the detector area is 1 m2, the recorded number ofparticles in a detector represents the particle density in thedetector.
The particle density in each detector is calculated by subtracting the concurrent pedestal value from the observed ADC count

33

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

15
10

38
40
42
58

a

a

400
300
Number of Eventsand by dividing it with the single particle gain for that detector.
The single particle calibration is performed periodically once in amonth for each detector [2]. To measure the dispersion in the response of the ﬁber- and the trapezoidal detectors, the following
measurements have been made by using four pairs of neighboring
detectors (one each of ﬁber- and trapezoidal-type). As mentioned
earlier in Section 1 the detectors are deployed on a hexagonal pattern. In the labeling scheme used each detector is assigned a number; the central detector is labeled 1 and the ﬁrst ring of 6 detectors
are labeled 2–7, second ring of 12 detectors are labeled 8–19 andso on. Therefore, a pair of successively numbered detectors are located closest to each other with a separation of only 8 m.
The fractional deviation in the particle density is calculated
from its mean value over the range of 3–20 particles m2. The four
detectors are used to determine the mean value at a given density.
Over this region (3–20 particles m2) of particle density, the PMT
response is linear for both types of detectors. The results of thisstudy are shown in Fig. 15, for the four pairs of detectors namely,(38,39), (40,41), (42,43), and (58,59). It is clear that the dispersion
in the response of the ﬁber detectors is relatively smaller, as compared to the trapezoidal detectors. Also the response of the four ﬁber detectors (38,40,42,58) are very similar to each other, and thefractional deviation in the particle density is [2.5%, whereas it is[5.5% for the trapezoidal detectors. However, even a variation of
5.5% is fairly small. Although, the response of the trapezoidal
detectors is found to be satisfactory, but the performance of the ﬁber detectors is comparatively superior.
As mentioned earlier in Section 1, an efﬁcient detector systemshould provide a large separation between the PMT noise andthe signal due to the charged particles in the EAS. In Fig. 16 theADC distributions obtained using muons passing through two
detectors namely; (a) #7 a trapezoidal shaped detector and (b)#10 a WLS ﬁber detector are shown. The separation between thepedestal and the single particle peak is signiﬁcantly larger in thecase of the new ﬁber detector (Fig. 16b) as compared to the older
trapezoidal detector (Fig. 16a).
The calibration of the low-gain PMT for a single particle is not a
viable option, because the PMT operating at such a low-gain would

200
100
0

b

400
300
200
100
0

50

100

150

200

250

300

350

400

Fig. 16. ADC distributions for passing muons for; (a) old trapezoidal shaped
detector #7, (b) new WLS ﬁber detector #10.

give too few ADC counts, to separate the single particle peak fromthe pedestal. Therefore, the calibration of the low-gain PMT is extracted indirectly, from the air shower data as detailed below. After
subtracting respective pedestals, a ratio of the high- and low-gain
PMT ADC counts is calculated for each air shower event. A mean of
this ratio (HLR) is calculated in steps of 50 ADC counts of the highgain PMT. The mean ratio ‘HLR’ is plotted as a function of the highgain PMT ADC counts, as shown in Fig. 17 for detector numbers 16,
42, 92, and 94. The ﬂat horizontal region in each plot, corresponds
to the common linear domain for both the high- and the low-gain
PMTs. This ﬂat region is ﬁt to a straight line parallel to the X-axis
and its intercept on the Y-axis provides, the ratio of the gains of
the high- and low-gain PMTs in the detector. Based on the data
shown in Fig. 17 a precision of about (1–2)% is obtained in the calibration of the low-gain PMTs relative to high-gain PMTs. However,this value is uncertain to about 5% due to the accuracy of the absolute calibration obtained from single muons for the corresponding
high-gain PMT.
At the lower particle densities, the low-gain PMT becomes relatively insensitive, while at higher densities the high-gain PMT
starts to saturate. Therefore, the ratio of the high-gain to the
low-gain ADC counts shows a reduction, both at small and large
particle densities. Using the single particle calibration for thehigh-gain PMT, and the extracted single particle gain as describedabove for the low-gain PMT, the observed integral particle density

5

-5

# 42

# 92

# 94

60

-10

10

# 16
80

40

Ratio of ADC Counts

Fractional Density Deviation (%)

0

39
41
43
59

b

5
0
-5

20

80
60
40

-10

20
5

10

15

20

Particle Density (m-2)
Fig. 15. Fractional variation in measured density as a function of detected particle
density: (a) ﬁber detectors 38, 40, 42, 58 (b) trapezoidal detectors 39, 41, 43, 59.
Detectors in each pair (38,39) (40,41), etc. are neighbors, located within ring #4 in
the array.

0

1000

2000

3000
4000
1000
2000
ADC Counts (high gain PMT)

3000

4000

Fig. 17. Ratio of ADC values for ‘high’ and ‘low’ gain PMTs as a function of high-gain
PMT ADC value. Each data point is the mean value of this ratio for 50 ADC channels.
Straight line is a ﬁt to the horizontal region in each plot.

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

spectrum for the GRAPES-3 experiment has been obtained. The
integral particle density spectra for detectors 18, 58, 96, and 100
are shown in Fig. 18. For the low-gain PMTs, the density spectrum
is shown only above 10 particles m2, as the value of the density israther uncertain below this threshold. In the case of the high-gain
PMT the response becomes non-linear above a density of 50 particles m2 and it saturates in the region of (100–300) particles m2.
On the other hand, the low-gain PMT response remains linear up to5000 particles m2 and no saturation is observed. Thus the combination of two PMTs allows the dynamic range of the ﬁber detector to be extended from 1 to 5000 particles m2. For most of thedetectors the gain ratio of the high- and the low-gain PMTs, is adjusted to be (20–100) with a mean value of 50. For determining
the density spectrum of the particles in the air showers, we haveused the data from the high-gain PMTs for the densities below
20 m2 and the low-gain PMTs for the densities above this value.
6. The density spectrum and EAS attenuation
The integral density spectrum of particle measured by the dualPMT ﬁber detectors has been obtained to validate the design ofthese new detectors. The limited dynamic range of the earlier trapezoidal shaped detectors had restricted the maximum detectableparticle density to only about 200 m2. However, with the newdual-PMT detectors this range has been extended to nearly 5000
particles m2 allowing meaningful measurements of density spectrum of particles in the EAS and their attenuation length.
The integral particle density spectra for four dual-PMT ﬁber
detectors namely 18, 58, 96, 100, respectively are shown in
Fig. 19. The successive spectra have been shifted downwards, bya factor of 10 for the sake of clarity. All four spectra display a powerlaw behavior and a ﬁt for the density range from 5 to 500 particles
m2, yields a spectral slope of (1.57 ± 0.04), consistent with theearly measurements [11,12]. These values are also close to theslope of the energy spectrum of the primary cosmic rays, as has
been pointed out in the past [11]. The nearly identical power law
spectral index for all four detectors, indicates that the dual-PMT
detectors are performing with a reasonably linear response, freefrom the effects of any saturation.
The zenith angle distribution of the EAS in different intervals ofshower size has been studied in the past, to measure the absorption of the EAS in the atmosphere for the primaries of different
energies. The ARGO–YBJ collaboration have argued that the measured particle density is due to the contribution of the showersa106

Number of EAS

34

d

6

10

4

10

2

18
58
96

-2

10

1

2

10
10
Particle density (m - 2 )10

3

Fig. 19. Integral density spectra for 4 dual-PMT detectors namely 18, 58, 96, 100,
respectively, for data from 1 March to 29 May 2005. Power law ﬁt to spectra from 5
to 500 particles m2 yields spectral index c = 1.57 ± 0.04. Successive plots shifted
downwards by factors of 10 for clarity.

within a very broad range of shower sizes, and the study of theangular distribution of the events with particle density exceedinga given value is considered, as appropriate as any other parametersuch as the shower size. It has been suggested that the angular distribution should display an exponential dependence. Following theequation (2) in reference [15],XIðP q; hÞ ¼ IðP q; 0Þ exp  ðsech  1ÞKa

ð4Þ

where, q is the particle density, h the zenith angle, X the verticalatmospheric depth, and Ka the attenuation length of the EAS forparticle density P q. Using the angular distribution of the EAS withdensity q P 3 particles m2, the ARGO–YBJ data could be ﬁt to Eq.
(4) up to a zenith angle h = 55°. A value of the attenuation length
Ka = (124 ± 11) g cm2 was derived for an observational depth of606 g cm2 [15]. The KASCADE collaboration has studied the attenuation of the EAS in great detail at the sea level as a function of theshower size and the zenith angle. They have obtained a value of theattenuation length Ka = 100–120 g cm2 [16]. Similar results havealso been reported by other groups [27,28].
The integral density spectra obtained, by combining the datafrom the 10 dual-PMT detectors taken over the 3 month interval,
are shown in Fig. 20 for 4 different zenith angle intervals namely;(a) 1.0 < sec h < 1.1, (b) 1.1 < sech < 1.2, (c) 1.2 < sech < 1.3 and (d)1.3 < sech < 1.4. Power law ﬁts to the data shown in Fig. 20, forthe density range 5–500 particles m2, for these four zenith angle

Number of EAS

Number of Events

10

c

10

1

104

6

8

100

b

102

10

10

8

10

7

10
10
10

6

5

(a)
(b)
(c)
(d)

4

104
10

3

102
1

10

10 2

10 3
1
10
10
Particle Density (m-2 )

2

10

3

10

4

10

2

1

10

10

2

10

3

Particle density ρ (m-2)
Fig. 18. Integral density spectrum from GRAPES-3 showers for high- and low-gain
PMTs of dual-PMT ﬁber detectors for data from 1 March to 29 May 2005. Detectors:(a) 18, (b) 58, (c) 96, and (d) 100. High-gain PMT shown by a dashed line and lowgain by continuous line.

Fig. 20. Integral density spectra using data from 10 dual-PMT detectors for 4 zenith
angle intervals; (a) 1.0 < sech < 1.1, (b) 1.1 < sec h < 1.2, (c) 1.2 < sech < 1.3 and (d)1.3 < sec h < 1.4.

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36

intervals are shown by dashed line. The magnitude of the spectral
index c shows a small, but gradual increase with the increasing zenith angle. The ﬁtted values of the spectral index c are 1.64, 1.65,
1.69, 1.71 for the four zenith angle intervals listed above. A similar
behavior has also been reported earlier [29].
The data for the same period of three months (1 March–29 May
2005) have been used to calculate the number of the EAS as a function of sec h  1 in steps of 0.02, where h is the zenith angle of theEAS. The dependence of the number of EAS on the function sec
h  1 is displayed in Fig. 21, for 4 different particle density thresholds namely; (a) > 5, (b) > 10, (c) > 20 and (d) > 40 m2. An excellent exponential ﬁt to the 4 sets of experimental data as shownin Fig. 21, is obtained using Eq. (4). The ﬁts are straight lines, due
to the logarithmic scale used for the Y-axis in Fig. 21. These ﬁts
are used to calculate the value of the attenuation length ‘Ka’ of
the EAS at the observational level.
To further study the attenuation of the EAS in the atmosphere,
the attenuation length ‘Ka’ is calculated using Eq. (4) and by varying the particle density threshold from 5 to 50 particles m2. The
values of Ka as a function of the particle density threshold ‘q’ isshown in Fig. 22. The value of Ka gradually decreases from106 g cm2 at q > 5 m2 to 98 g cm2 at q > 50 m2. The range of
Ka = 98–106 g cm2 reported here for an atmospheric depth of
800 g cm2 at Ooty is comparable to the measurement of the
ARGO–YBJ collaboration of Ka = (124 ± 11) g cm2 for an atmospheric depth of 606 g cm2 in Tibet [15] and Ka = 100–120 g cm2by the KASCADE collaboration [16].

35

7. Conclusions
The performance of the newly developed plastic scintillator
detectors with WLS ﬁber readout used in the expansion of the
GRAPES-3 experiment has been described in detail. The detector
performance in terms of the photon output, the uniformity andthe time resolution are in good agreement with the results of theMonte Carlo simulation code G3ScSIM. The use of the WLS ﬁbers
has allowed a sizable increase in the photon yield and has also resulted in greater uniformity in the detector response. As a result of
this study and due to the simplicity of the fabrication procedure,
the parallel groove conﬁguration for the layout of the WLS ﬁbers
has been used. This resulted in a signiﬁcant improvement in theperformance of the ﬁber detectors as compared to the old trapezoidal detectors. The dynamic range of the dual-PMT ﬁber detectors
for the measurement of the particle density has increased to5  103 particle m2. This enhanced dynamic range covers theparticle densities encountered near the cores of the extensive air
showers of energy well above the ‘knee’ in the spectrum. About
100 dual-PMT ﬁber detectors are currently operating in the arrayand more are being continuously added.
The integral density spectrum of the dual-PMT ﬁber detectors
displays a spectral slope c = 1.57 ± 0.04. This value is consistent
with the measurements reported earlier in the literature. The zenith angle dependence of the density spectra has been used to estimate the attenuation length ‘Ka’ of the EAS in the atmosphere. The
variation in Ka, as a function of the particle density threshold has
been calculated for the range 5–50 particles m2. The attenuation
length Ka is found to vary in the range 98–106 g cm2, for an atmospheric depth of 800 g cm2 at Ooty. The calculated value of Ka isalso consistent with the measurements reported earlier.

14

Acknowledgement(a)Loge (N)

12

(b)
10

(c)
(d)

8
0

0.1

0.2
sec θ - 1

0.3

0.4

Fig. 21. Number of EAS as a function of sec h  1 for 4 particle density thresholds;
9(a) > 5, (b) > 10, (c) > 20, and (d) > 40 m2. Straight line ﬁts obtained from Eq. (4).

We thank D.B. Arjunan, K. Manjunath, S. Murugapandian, B. Rajesh, C. Ravindran, K. Santosh Kumar and R. Suresh Kumar for their
help in the testing, installation and operation of the detectors andthe electronic modules. The administrative services of V. Viswanathan during the experiment are acknowledged. We thank A.A. Basha, G.P. Francis, I.M. Haroon, V. Jeyakumar, K. Ramadass, for their
help in the fabrication, assembly and the installation of various
mechanical components and detectors. The Japanese members of
the GRAPES-3 collaboration acknowledge the partial ﬁnancial support from the Ministry of Education and Science of the Governmentof Japan for the experiment.

References110Λ (g.cm-2)

105

100

95

5

10

ρ

20

40

Fig. 22. Variation of the attenuation length Ka as a function of the particle density
threshold q in the range 5–50 m2.

[1] S.K. Gupta et al., Phys. Rev. D68 (2003) 052005.
[2] S.K. Gupta et al., Nucl. Instr. Meth. A 540 (2005) 311.
[3] Y. Hayashi et al., Nucl. Instr. Meth. A 545 (2005) 643.
[4] T. Nonaka et al., Phys. Rev. D74 (2006) 052003.
[5] P. Subramanian et al., Astron. Astrophys, in press.
[6] V.I. Kryshkin, A.T. Ronzhin, Nucl. Instr. Meth. A 247 (1986) 583.
[7] D.A. Bukin et al., Nucl. Instr. Meth. A 384 (1997) 360.
[8] S.R. Borenstein et al., IEEE Trans. Nucl. Sci. NS-29 (1982) 402.
[9] A.D. Bross, A. Pla Dalmau, Nucl. Instr. Meth. A 325 (1993) 168.
[10] M. Adams, et al., Nucl. Instr. Meth. A 366 (1995) 263; V. Hagopian, Nucl. Phys.
B (Proc. Suppl.) 61 (1998) 355.
[11] K. Greisen, Ann. Rev. Nucl. Sci. 10 (1960) 63.
[12] I. Katsumata, J. Phys. Soc. Jpn. 19 (1964) 800.
[13] R.W. Clay, P.R. Gerhardy, Il Nuovo Cimento 4 C (1981) 26.
[14] D. Ciampa, R.W. Clay, J. Phys. G: Nucl. Phys. 14 (1988) 787.
[15] C. Bacci et al., Astropart. Phys. 17 (2002) 151.
[16] T. Antoni et al., Astropart. Phys. 19 (2003) 703.
[17] Kuraray optical products, Nihonobashi, 1-6,3-Chome, Chuo-ku, Tokyo, Japan.
[18] http://www2.dupont.com/Tyvek/en-US/assets/downloads/tyvekhandbook.pdf.
[19] P.K. Mohanty et al., TIFR/DHEP, Preprint (2008).

36

P.K. Mohanty et al. / Astroparticle Physics 31 (2009) 24–36[20] L.D. Landau, J. Exp. Phys. (USSR) 8 (1944) 201.
[21] D.H. Wilkinson, Nucl. Instr. Meth. A 383 (1996) 513.
[22] K.S. Kolbig, B. Schorr, Comput. Phys. Commun. 31 (1984) 97.
[23] M. Aguilar-Benitez et al., Phys. Lett. B 170 (1986) 1.
[24] N.S. Kapany, Fiber Optics: Principles and Applications, Academic Press, London
and New York, 1967.

[25] W.L. Leo, Techniques for Nuclear and Particle Physics Experiments, Narosa
Publishing House, New Delhi, 1995.
[26] M. Moszynski, Nucl. Instr. Meth. A 158 (1979) 1.
[27] D.E. Alexandreas et al., Nucl. Instr. Meth. A 328 (1992) 570.
[28] M. Aglietta et al., Nucl. Phys. B (Proc. Suppl.) 70 (1999) 509.
[29] J.D. Hollows, H.W. Hunter, A.N. Suri J. Phys. A: Gen. Phys. 2 (1969) 591.